Q4. Zinc is extracted by heating zinc oxide with coke: ZnO(s) + C(s) → Zn(s) + CO(g). Identify the substances oxidised and reduced, and the oxidising and reducing agents.
Carbon gains oxygen, so it is oxidised and is the reducing agent. Zinc oxide loses oxygen, so it is reduced and is the oxidising agent.
Q5. In 2FeCl3(aq) + H2(g) → 2FeCl2(aq) + 2HCl(aq), ferric chloride is reduced. What is oxidised, and why?
Hydrogen. Ferric chloride loses the electronegative element chlorine, which is added to hydrogen to form HCl; adding an electronegative element is oxidation, so hydrogen is oxidised (and is the reducing agent).
Q6. In the combustion of methane, CH4 + 2O2 → CO2 + 2H2O, which reactant is oxidised and which is reduced? Give two classical reasons for the first.
Methane is oxidised: its carbon gains oxygen, and it loses its hydrogen. Dioxygen is reduced, because hydrogen (electropositive compared with oxygen) is added to it to form water.
Q7. Why did chemists coin the single word “redox” instead of speaking of oxidation reactions and reduction reactions separately?
Because oxidation and reduction always occur together: electron loss by one species is balanced by electron gain by another; the classical atom-transfer examples illustrate this pairing. For example, in 2HgCl2 + SnCl2 → Hg2Cl2 + SnCl4, stannous chloride is oxidised while mercuric chloride is reduced.
Q8. Calcium combines with hydrogen to form calcium hydride, Ca + H2 → CaH2. Using electronegativity, decide which element is oxidised. Why is the rule “gain of hydrogen is reduction” misleading here?
Hydrogen is more electronegative than calcium, so it plays the part of an electronegative element: calcium is oxidised and hydrogen is reduced. The rule “gain of hydrogen is reduction” assumes hydrogen is the electropositive partner, which is not true with reactive metals such as Na and Ca.